Conventionally, various speech encoders applicable to digital mobile communication systems have been proposed and practically used in, for instance, the car industry. A CELP (Code Excited LPC coding) process is typically used ill the systems.
The CELP process is a speech encoding process in which an excitation signal of speech is generated by a codebook, wherein short term parameters representing spectrum characteristics of a speech signal are sampled from the speech signal in each frame of, for instance, 20 ms, and long term parameters representing pitch correlation with the past speech signal are sampled from the presently supplied speech signal in each subframe of, for instance, 5 ms. Thus, long and short term predictions are carried out to obtain long and short term excitation signals by the pitch and spectrum parameters, so that a synthesized speech signal is generated by adding the long term excitation signal to a signal selected from a codebook storing predetermined kinds of noise signals (random signals), and then adding the short term excitation signal to the signal thus obtained in the above addition of the long term excitation signal to the codebook selected signal. This synthesized speech signal is compared with an input speech signal in a subtractor to generate an error signal, so that one kind of noise signal is selected from the codebook to minimize the error signal. This CELP process is described in a report titled "Code-excited linear prediction: High quality speech at very low bit rates" by M. Schroeder and B. Atal on pages 937 to 940 "ICASSP, Vol. 3, March 1985".
In this CELP process, a VSELP (Vector Sum Excited Linear Predication) process has been proposed. Between the both processes there is a difference in that a synthesized signal is generated in the VSELP process by the linear coupling (code summation) of more than two predetermined basis vectors, so that the synthesizing process steps are largely decreased in number to improve error tolerance as compared to the CELP process.
In the VSELP process, the linear coupling of optimum basis vectors is transmitted from a transmitting side to a receiving side by using parameters defined codewords. For this purpose, optimum codewords must be searched on the transmitting side. This search is defined "codebook search". A conventional codebook search system is described in the U.S. Pat. No. 4,817,157, as explained later.
However, the conventional codebook search system has a disadvantage in that the number of functions to be used for computing cross correlations is large, resulting in difficulty of addressing and an increase in amount of calculations necessary for realizing a hardware system using signal processing LSIs (DPSs).